1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
In conventional wireless telecommunications, one or more mobile units (or access terminals) may establish a wireless link to a Radio Access Network (RAN). The RAN architecture is typically hierarchical and call state information associated with each mobile unit call session is stored in a central repository, such as a Radio Network Controller (RNC), a Packet Data Serving Node (PDSN), and the like. One alternative to the conventional hierarchical network architecture is a distributed architecture including a network of base station routers. For example, each base station router may combine RNC and/or PDSN functions in a single entity that manages radio links between one or more mobile units and an outside network, such as the Internet. Compared to hierarchical networks, distributed architectures have the potential to reduce the cost and/or complexity of deploying the network, as well as the cost and/or complexity of adding additional wireless access points, e.g. base station routers, to expand the coverage of an existing network. Distributed networks may also reduce (relative to hierarchical networks) the delays experienced by users because packet queuing delays at the RNC and PDSN of hierarchical networks may be reduced or removed.
In a distributed architecture, one or more mobile units may establish a call session with any one of the plurality of base station routers. Accordingly, each base station router should be capable of assigning an identifier, such as a Unicast Access Terminal Identifier (UATI), to the mobile unit. For example, a proposed Code Division Multiple Access (CDMA) protocol standard, sometimes referred to as the EVolution-Data Only (EVDO) standard, specifies a unique 128-bit UATI that is assigned to a mobile unit when a call session is initiated by the mobile unit. The mobile unit maintains the UATI for the duration of the call session. In the current implementation, the EVDO call session UATI is divided into two parts: a 104-bit UATI104 and a 24-bit UATI024. The UATI024 portion is unique to the mobile unit for the duration of the call session and the UATI104 is common to all mobile units within a predetermined subnet of base station routers in the distributed network.
In operation, base station routers in a conventional distributed network may broadcast, or advertise, their subnet address, e.g. the address indicated by the UATI104 portion of the UATI. However, the address is generally too long for mobile units to insert in the user identification portion of each message sent to the base station routers, so the base station routers also typically advertise a color code for use by the mobile station, which is typically an 8-bit representation of (or alias to) the 104-bit UATI104 subnet address. The base station router providing service to the mobile unit may change, e.g., due to fluctuating channel conditions and/or when the mobile unit roams between different subnets in the distributed network. Accordingly, mobile units may determine whether or not the subnet including the base station router providing service to the mobile unit has changed by monitoring the advertised subnet address on the control channel.
Mobile units typically request a call session transfer from the old subnet to the new subnet when the mobile unit detects a change in the subnet address. For example, a mobile unit may initiate a call session with a first base station router belonging to a first subnet having a first subnet address. The first base station router assigns a UATI to the mobile unit. When the mobile unit becomes associated with a second base station router belonging to a second subnet having a second subnet address, the mobile unit may request, using the associated color code, that its previous call session information be transferred from the first base station router to the second base station router. The color code may be used by the base station router to locate the first subnet when the mobile unit is able to determine how it made the transition from the first subnet to the second subnet. For example, when an idle mobile unit roams from the first subnet to the second subnet, which neighbors the first subnet, the color codes uniquely identify the first and second subnets and so they may be used to locate the first subnet.
However, mobile units are not always able to determine how they made a transition from one subnet to another. For example, coverage holes, temporary loss of the radiofrequency carrier, powering down the mobile unit during travel, and other conditions may prevent the mobile unit from determining how a transition was made between subnets. When the mobile unit is not able to determine how a transition was made between subnets, the mobile unit cannot ensure that the color codes uniquely identify the old and/or new subnets. For example, if a user powers down a mobile unit before traveling from New York to Los Angeles, the mobile unit cannot guarantee that the color code associated with the old subnet in New York will uniquely identify the old subnet to a base station router in Los Angeles. To the contrary, it is very likely that the base station router in Los Angeles would interpret the color code associated with the old subnet as indicating a different subnet in the Los Angeles area.
Idle mobile units are not typically able to request a call session transfer using only the color code when the mobile unit cannot determine how it transitioned from one subnet to another. Instead, the mobile unit initiates a new call session with the new base station router in the new subnet and then attempts to locate the previous call session using the complete subnet identifier, e.g. the UATI assigned by the originating base station router. For example, in 1xEVDO wireless systems, idle mobile units that cannot determine how a subnet transition was made typically create a new session with the base station router in the new subnet. A connection between the mobile unit and the base station router is then formed and then the mobile unit may negotiate transition of the call session associated with the originating base station router to the base station router in the new subnet using the full 128-bit UATI. This technique is conventionally referred to as the Prior Session method of inter-subnet idle hand-off.
Idle mode hand-off techniques such as the Prior Session method have a number of disadvantages. For example, idle mode hand-offs typically occur in border regions near the edges of coverage areas associated with base station routers and/or subnets. The border regions are frequently areas of poor and/or fluctuating radio frequency coverage, at least in part because they may be relatively distant and/or obscured from the base station router. Consequently, mobile units in the border regions may be frequently and/or rapidly handed off between different base station routers. In some cases, the handoff frequency may be so rapid that the mobile unit may initiate a call session transfer using the Prior Session method after being handed off from a first base station router to a second base station router, only to be transferred back to the first base station router before the call session transfer is complete. However, the mobile unit cannot resume its previous call session with the first base station because the mobile unit has already initiated the Prior Session method. Instead, the mobile unit must request a prior session from the second base station router, resulting in session churn. In some cases, the older session associated with the first base station router is not immediately removed and may persist for a time period specified by the system operator, which may be as long as several hours, or the standards default duration of 54 hours. An accumulation of these older sessions may reduce the number of available call sessions and artificially inflate session usage. When the system is heavily loaded, the accumulated older stranded sessions may block the creation of new sessions.